From the prior art, hybrid vehicles that comprise a hybrid transmission containing a main transmission and a hybrid assembly are known, in which the hybrid assembly as a rule replaces a hydrodynamic torque converter. In addition to the internal combustion engine they comprise at least one electric motor coupled to the main transmission by a gear system, as a rule, in the form of a planetary gear system, or an electric machine which can be operated either as a motor or as a generator, depending on the operating situation.
In series hybrid vehicles, a generator is driven by the internal combustion engine, the generator supplying electrical energy to the electric motor that drives the wheels. In such a case as a rule no gear steps are needed, since the electric machines used have a sufficiently large rotational speed range and high enough starting torque to move the vehicle. However, in series hybrid vehicles the vehicle is never driven purely mechanically by the internal combustion engine since no provision is made for coupling the internal combustion engine to the drivetrain, with the result that the drive energy is transmitted via an electric path which is characterized by greater losses compared with mechanical transmission. On the other hand, when wheel-hub motors are used in articulated buses a multi-axle drive can be created, which ensures an advantageous distribution of the drive power between the front coach and the trailer.
Furthermore parallel hybrid vehicles are known, in which the torques of the internal combustion engine and at least one electric machine that can be connected to the internal combustion engine are added, preferably by means of a summation gear system such as a planetary gear system. In this case the at least one electric machine can be connected to the belt drive or to the crank shaft of the internal combustion engine. The torques produced by the internal combustion engine and/or the at least one electric machine are transmitted to the drive axle by a downstream transmission.
For example, DE 103 46640 A1 by the present applicant describes a drivetrain for a hybrid vehicle, in which an internal combustion engine is in driving connection with a separator clutch whose output side is connected to a transmission input shaft connected to an input element of an automatic transmission. In the known drivetrain the transmission input shaft can also be driven by an electric machine, and for this purpose the rotor of the electric machine is connected to the transmission input shaft.
Parallel hybrid vehicles usually have an electric machine arranged on the primary side. When the main transmission is a powershift transmission, recuperation can also take place during a gearshift, i.e. electrical energy can be recovered from mechanical energy. When a semi-automatic manual-shift transmission is used as the main transmission the transmission has to be shifted under no load to carry out a gearshift, and this results in the disadvantage that the recuperation with the electric machine has to be interrupted.
In articulated buses, powershift automatic transmissions are predominantly used, which besides greater comfort and higher drive-power also offer potential savings compared with semi-automatic shifts with traction force interruption, since during a powershift, rotation energy stored in the primary masses is transferred into the drivetrain as a dynamic fraction of the drive torque. Disadvantageously, powershift transmissions have lower efficiency than a semi-automatic manual-shift transmission and are more expensive in terms of both production and maintenance.